1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus.
2. Description of the Background Art
It is a common practice with an image forming apparatus to uniformly charge the surface of a photoconductive drum or image carrier, scanning the charged surface of the drum with a light beam in accordance with image data to thereby form a latent image, and develop the latent image with toner to thereby produce a corresponding toner image. The toner image is then transferred from the drum to a sheet or recording medium. After the image transfer, toner left on the drum is removed.
While discharge is, in many cases, used for the purpose of uniformly charging the surface of the drum, the problem with discharge is that it generates nitric acid compounds. When nitric acid compounds are coupled with moisture present in air, there are generated nitric acid, nitrate and other ionic substances. Further, such ionic substances react with ammonia present in air, generating ammonium nitrate.
When the ionic substances and ammonium nitrate, which are apt to deposit on the drum, deposit on the drum, they accelerate the deterioration of the drum to thereby bring about the unusual wear of the drum. Further, the ionic substances make the surface of the drum electrically conductive, causing a latent formed on the above surface to blur or run.
Toner filming to occur on the surface of the drum is another causes of a defective image. More specifically, first, a wax component and a resin component contained in toner deposits on the drum surface, and then paper dust, which easily absorbs moisture, deposits on the wax component and resin component. Subsequently, the ionic substances deposit on the paper dust absorbed moisture, forming a film on the drum surface. Such toner filming is likely to cause an image formed on the drum to run.
In light of the above, it has been customary to cause the surface of the drum to wear with a cleaning blade, which contacts the above surface, for thereby removing the ionic substances, ammonium nitrate and toner filming. This, however, brings about another problem that because the surface layer of the drum is worn by 2 μm to 3 μm for 100,000 rotations, the durability and therefore the life of the drum is reduced although image quality may be enhanced.
To solve the above problem, the surface layer of the drum may be increased in hardness and wear resistance to reduce the amount of wear for thereby increasing durability, as proposed in the past. However, it is difficult to sufficiently shave oft the ionic substances, ammonium nitrate and toner filming from such a hard, highly wear-resistant surface and therefore to sufficiently cope with the unusual wear of the above surface and image running. As a result, image quality is lowered.
In the circumstances described above, when use is made of a drum whose surface layer is increased in hardness and wear resistance for enhancing durability, some special means for removing the ionic substances, ammonium nitrate and toner filming from the above surface layer must be used. For example, means for increasing the pressure of a cleaning blade acting on the drum at preselected timing is essential. It is difficult to arrange such special means in a limited space available around the drum.
On the other hand, an electrophotographic image forming apparatus in which a drum heater is accommodated in a drum for heating the drum has been proposed in order to obviate image running particular to the highly durable, highly wear-resistant drum surface. Although heating the drum is successful to reduce image running, the diameter of the drum must be made large enough to accommodate the drum heater. This is contradictory to the current trend toward the size reduction of an electrophotographic image forming apparatus.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Publication No. 6-82234 and Japanese Patent Laid-Open Publication Nos. 7-72770, 7-225541 and 2000-321806.